1. Field of the Invention
The present invention relates to an electric power steering system, and more particularly, to a control apparatus suited to control a motor of the electric power steering system without directly detecting a current of the motor.
2. Description of the Related Art
There is known a conventional electric power steering system that drives a motor while performing feedback control so that a motor target current calculated on the basis of a steering torque signal sent from a torque sensor coincides with a measured value of a current of the motor (e.g., JP 2001-206232 A).
There is also known another conventional electric power steering system that drives a motor on the basis of a detection signal of a steering speed and a steering torque signal without measuring a current of the motor (e.g., JP 61-169368 A).
There is also known still another conventional electric power steering system that drives a motor while performing feedback control of an estimated value of a current of the motor on the basis of a steering torque signal without measuring the current of the motor (e.g., JP 3714843 A).
Electric power steering systems, which have started its practical use as a power steering system for light passenger vehicles, have recently been applied further to ordinary passenger vehicles with heavy vehicle weights, and attempts to increase the current and output of motors of these electric power steering systems are under way. With this background, in order to detect the current of the motor using an inexpensive resistance and perform feedback control as in the case of the related art disclosed in JP 2001-206232 A, an interface circuit for inputting to a microcomputer a signal detected using the resistance is required in addition to an element of this resistance. Therefore, there is a problem in that the area of a substrate of a control apparatus needs to be increased so as to mount the interface circuit and the element of the resistance. Further, this resistance generates a large amount of heat, and hence a heat sink fitted to the control apparatus to discharge this heat needs to be enlarged. As a result, there is another problem in that enlargement of the apparatus and cost rise are incurred. In addition, there is still another problem in that a loss of power by this resistance hinders the attempt to increase the output of the motor when the current of the motor is detected using the resistance as described above.
On the other hand, with a view to suppressing the generation of heat, the loss of power, and enlargement of the apparatus resulting from the resistance for detecting the current of the motor, it is conceivable to control the current of the motor without carrying out detection thereof. For example, JP61-169368 A discloses a method in which a back electromotive force of the motor is compensated for on the basis of a steering speed detection signal to control the current of the motor in an open loop. Further, JP 3714843 A discloses a method in which the current of the motor, which is estimated from a drive voltage of the motor, is subjected to feedback control.
In general, the influences of dispersion of the resistance of windings of a motor and temperature characteristics thereof can be suppressed if a current of the motor is detected and subjected to feedback control. However, when the current of the motor is not detected as in the cases of these conventional systems, the torque of the motor changes as a result of a likelihood of changes in the current of the motor, and hence the steering torque of a driver is influenced. In these conventional systems, a disturbance in the voltage of the motor such as noise or the like is also more likely to lead to a fluctuation in the current of the motor than in cases where the current of the motor is detected. As a result, the influences of vibrations, noise, and the like are not negligible.